This invention relates to a method, and a device for continuous centrifugal classifying of a continuous flow of particulate material and more particularly such a method permitting classification into at least one fraction of coarse material and at leat one fraction of fine material in a deflected flow, either in a gaseous fluid at cut-off sizes between approximately 1 .mu.m and 100 .mu.m, if the mass flow ratio of the supplied material to the flow of classifying gas is up to approximately 10, or in a liquid fluid at cut-off sizes between approximately 10 .mu.m and 1 mm, the Reynolds number related to the radial transverse extension of the classifying flow being greater than 2,000. The Reynolds number is defined as: EQU Re=v .multidot.d/.nu.
in which v is the speed of the fluid,
.nu. is the kinematic viscosity of the fluid, and PA1 D is the radial transverse extension of the deflected flow.
In some known classifying methods and devices, separation occurs in a flow deflected by walls. The most well known and widely used application, which also applies to the separation of material uniformly divided in a flow of fluid, is deflection classification in a deflection or "slat" classifier. Slat classifiers are used, for example, in oval fluid energy mills. Another embodiment of a slat classifier is described in U.S. Pat. No. 3,006,470. In slat classifiers, the fluid, uniformly charged with the material for sifting, flows in a channel which is usually straight. After leaving the channel, some of the fluid is sharply deflected by a lateral slat system comprising a relatively large number of parallel slats forming parallel outflow channels between them, and is thus discharged. The deflected fluid entrains the fine material, whereas the coarse material remains in the fluid, which flows in straight lines. The front edges of the slats are relatively sharp. Consequently, the flow is deflected around relatively sharp edges having a radius of curvature which is very small compared with the dimensions of the straight channel and the entire length of the slats in the flow direction. The material for sifting is relatively uniformly distributed in the inflow channel. Owing to these characteristics of deflection classifying in a slat classifier, the selectivity is relatively low for classifying below 100 .mu.m and relatively high loads of material. The relatively sharp deflection also necessitates a high pressure drop, i.e., a high energy requirement. The deflected flow in a slat classifier is a curved non-parallel flow which separates at the sharp deflection edges. In the flow, similar particles of material move along different trajectories, depending on the centrifugal force exerted on them in accordance with the initial position.
It is also known for classification to occur in flows which are not curved by deflecting walls or guided along walls. Such classification is performed, for example, in "spiral air classifiers" having a housing which is annular in cross-section and in which an axially symmetrical flow is maintained in an inward spiral. Such a method, therefore, is not comparable with deflection classification. In the case of spiral air classification, the fine material in the curved spiral flow moves inwards whereas the coarse material flows outwards, relative to the curved flow, towards the outer wall of the classifier housing, and is then removed. Spiral flow is suitable for fine classifying and is widely used for that purpose, but has a serious disadvantage in that particles of material which are at or near the cut-off size accumulate in the classifying chamber as a result of the equilibrium between the outward centrifugal force and the inward extraining force and, as a result of the concentration gradient, are diffused and discharge partly with the coarse material and partly with the fine material, thus reducing the selectivity. Since the classifier flow charged with fine material emerges axially from the classifier chamber, there are limitations to the axial width of the chamber and the throughput.
A disadvantage common to slat classifiers and spiral air classifiers is that the material can be separated into only two fractions.
Deflection classifying must also be distinguished from cross-current classifying as disclosed in British Pat. No. 1,088,599 and the corresponding U.S. Pat. No. 3,311,234, and British Pat. No. 1,194,213 and the corresponding U.S. Pat. No. 3,520,407 and the Canadian Pat. No. 834,558 in which the material is introduced at a given initial speed into a flow extending at an angle or almost in the opposite direction. In such a separator the coarse material travels through the flow. On the other hand, the particles of fine material are decelerated and deflected in the flow, the deceleration distance and the acceleration distance in the flow direction both depending on the particle size. These classifiers are unsuitable for very fine separation. This is clear from the fact that the deceleration distance of a particle of material having a diameter of 10 .mu.m and a density of 1 g/cm.sup.3 is only 5 mm in stagnant air at an initial speed of 30 m/sec. Counter-current and cross-current classifiers of this kind are not centrifugal classifiers in which the particles of material suspended at the center of the flow are subjected to centrifugal force owing to the curvature of the flow. Instead, they are deflected in the flow to an extent depending on their size, but only because their entry speed differs from that of the flow.